1. Field of the Invention
The present invention relates to an optical disk of a system wherein a disk substrate, a recording layer and a thin film cover layer are laminated in this order, and the recording layer is to be irradiated through the thin film cover layer with a laser beam to be used for recording and retrieving data, wherein on the surface of the thin film cover layer, a hard coat layer is further formed which is excellent in abrasion resistance, transparency and long-term sebum stain proofness, particularly removability of fingerprint attached to its surface.
2. Discussion of Background
In recent years, attention has been drawn to an optical recording device capable of recording a large amount of data quickly in high density and capable of quickly retrieving the recorded information, in response to the progress in multimedia. Such an optical recording device includes one for retrieving information recorded on a read-only disk having data preliminarily stamped on a disk at the time of preparation of the disk to permit only to retrieve the information, such as a compact disk (CD) or a laser disk (LD), one for recording data and retrieving the data recorded on a recordable disk capable of recording only once, such as CD-R, and one for recording data and retrieving the data recorded on a rewritable disk capable of rewriting and erasing data as many times as desired by means of a magnetooptical recording system or a phase change recording system. By these optical recording devices, recording and retrieving of data are carried out by means of a beam spot having a laser beam narrowed down to a diffraction limit by a lens. The size of this beam spot will be about λ/NA where λ is the wavelength of the laser beam, and NA is the numerical aperture of the lens (“Basic and Application of Optical Disk Storage”, compiled by Yoshihito Kakuta, Institute of Electronics, Information and Communication Engineers, 1995, p 65).
In order to record information in a higher density i.e. to form a smaller pit pattern on an optical recording medium, it is necessary to make a beam spot smaller. In order to reduce the size of the beam spot, two methods are conceivable from the above formula i.e. a method of shortening the laser beam wavelength (λ), or a method of increasing the numerical aperture (NA) of the lens. The wavelength of a semiconductor laser for an optical disk which is commonly used at present, is mainly from 780 to 680 nm, but a study is being made to employ an orange color laser having a shorter wavelength of 650 nm, or a green or blue laser beam having a further shorter wavelength.
Particularly, as a system of employing a blue laser, it has been proposed to obtain a higher recording density by adjusting the wavelength of the light source to a level of 400 nm and NA to at least 0.6. However, the allowance for an angle (a tilt angle) of the optical disk plane deviated from the right angle to the optical axis and the allowance for non-uniformity in thickness of the optical disk decrease due to shortening of the wavelength of the light source or increase of NA of the objective lens.
The reason for the decrease of such allowances is such that in the case of the tilt angle of the optical disk, coma aberration occurs, and in the case of the non-uniformity in thickness of the optical disk, spherical aberration occurs, whereby the condensing performance of the optical head device deteriorates, and read out of a signal tends to be difficult.
In a conventional compact disk (CD) or the like, the thickness of a cover layer which can be formed on the surface of a recording layer is 1.2 mm, and in a digital versatile disk (DVD) or the like, the thickness of such a cover layer is 0.6 mm, whereby the respective disk substrates themselves have played the role of such cover layers, and the recording layer has been irradiated with a laser beam through the disk substrates. However, in the system of employing a blue laser, not only the numerical aperture (NA) of the lens is made large, but also in order to increase the allowance for the above tilt or the allowance for the non-uniformity in thickness of the optical disk, it is necessary to make this cover layer thin to a level of 0.1 mm. Accordingly, in the blue laser system, it has become impossible to let the substrate itself play the role of the cover layer, like in the conventional optical disk. Therefore, it is necessary to form a recording layer made of a laminated film comprising a reflective film, a recording film and the like on the substrate and to form a thin film cover layer with a thickness of about 0.1 mm on the surface of the recording layer, and the recording layer will be irradiated with a laser beam through the thin film cover layer.
When a laser having a short wavelength like a blue laser is used, the distance between the optical head and the optical disk will be small at a level of from 0.1 to 0.2 mm. In an optical recording device, in order to accomplish a high data transfer rate, an optical disk is required to be rotated at an extremely high speed, and depending upon the combination of the wavelength of the laser beam to be used, the numerical aperture (NA) of the lens, the recording capacity of the disk and the desired data transfer rate, the maximum rotational speed of an optical disk may reach a level of at least 2,500 rpm, in some cases at least 5,000 rpm. As an optical disk is rotated at a high speed in the state where the distance of the optical head and the optical disk is very narrow, it is possible that the optical head will be in contact with the optical disk, and accordingly, the thin film cover layer covering the surface of the recording layer, is required to have a high abrasion resistance.
Further, when a laser having a shorter wavelength is used, scratches formed or dust deposited on a laser beam incident surface of the optical disk is likely to lead to errors at the time of recording or retrieving. Therefore, in order to prevent such errors at the time of recording or retrieving, the thin film cover layer constituting the laser beam incident surface is required to have higher abrasion resistance.
Further, not only scratches or dust deposited on the surface but also attachment of fingerprints or sebum resulting from handling by a user are likely to lead to errors at the time of recording or retrieving. In order to prevent such errors at the time of recording or retrieving, the thin film cover layer constituting the laser beam incidence surface is required to have not only abrasion resistance but also stain proofness against fingerprints and sebum.
Many attempts have been made to prevent attachment of stains such as fingerprints and sebum, and many of them are to employ a fluorine-containing compound. These attempts have been realized by achieving a surface which is likely to repel higher fatty acids and their esters, etc. as components of stains in common. Further, in order to maintain the effect of preventing attachment of stains such as fingerprints and sebum, such attempts have been made as introduction of a hydrolysable silyl group into a molecule of the fluorine-containing compound (JP-A-11-217558) and introduction of a crosslinking moiety with a resin forming the hard coat layer (JP-A-11-293159).
However, such a fluorine-containing compound has such a drawback that it is hardly compatible with a resin forming the hard coat layer, and when added as one composition, it is likely to impair transparency of a cured product. Further, in a case where the fluorine-containing compound is separately coated on the surface of the disk (JP-A-11-213444), since the fluorine-containing compound itself has no mechanical strength, it is required to coat it on the hard coat layer to prevent scratches on the disk surface, and thus the procedure will be complicated.